“Fatigue Analysis of a Harsh Environment FPSO using SESAM”

Scope of Presentation

• Typical harsh environment FPSO

• Identification & classification of fatigue sensitive locations

• Outline of analysis procedure

• Example using SESAM

• Performance

Terra Nova – Harsh Environment FPSO

Key Questions!

• What locations ?

• What analysis procedure ?

• What software ?

• What model ?

• What vessel condition ?

• How is procedure implemented ?

• How can performance be improved ?

Fatigue Sensitive Locations

Main hull

Mooring system

Flare tower Crane pedestals

Openings Module supports

Hopper knuckles

Turret structure

Main bearing points Mooring line connection

Shell connections

DnV Classification Note CN30.7

Design Wave Approach Sec. 5.5

Equivalent Long Term Stress Distribution (Weibul

param.), Sec. 5.2

Long Term Stress Distribution, Sec. 3.2

FE model of detail, Sec. 6.3-6.6

Stress Component based Stochastic Fatigue Analysis

Sec. 5.6

Simplified Analysis Direct Analysis

Load Response Sec. 4.2-4.4 Load Response Sec.5.2 Load Transfer Function. Sec. 5.3

2.2 Stress Components Interchangeable Results

FE model of ship, Ch.6

SCF: K-factors,Ch.7 Interchangeable Results

Combination of Stresses, Sec. 3.4-3.4 Local Stress Transfer Functions

for stress components Sec. 5.4

Full Stochastic Fatigue Analysis Sec. 5.7

Fatigue Damage Summation:

Summation of damage contributions from each wave period/ship heading combination for each sea state in the

wave scatter diagram

Fatigue Damage Calculation, Sec. 2.1

Software

• Repetitive hull geometry makes it ideally suited to superelement approach.

• Sub-modelling can be used where necessary. Hot spots do not have to be known a priori.

• Totally integrated solution. All pre- & post- processors necessary for complete analysis are available.

• Committed software support

SESAM is the Preferred Tool for Fatigue Analysis of FPSO’s because . . .

Hydrodynamic Analysis Model Hull Form Example

Structural Analysis Model Moonpool Fatigue

Structural Analysis Model Mid-ship and Wingtank Superelements

Structural Analysis Model Moonpool Superelements

Structural Analysis Model Turret Superelement

Structural Analysis Model Typical Mid-ship Section Showing Scantlings

Implementation

POSTRESP Calculate fatigue lives

POSTFEM Extract stresses Review behaviour Select SN curves

Stage 1

Obtain vessel mass and geometry data

Obtain moonpool geometry data

Establish locations to be analysed & level of modeling detail

WADAM Create motions model

Stage 3

PREFEM Create Geometry model

WADAM Create RAO’s

PREFEM Apply loads & boundary conditions

SESTRA Obtain Unit Stresses

Stage 4 Stage 2

PRESEL Assemble superelements

PREPOST Create results database

Implementation

1 2 3 4 5 6 7 8

Apply Unit Load Cases

Multiply by ‘Wadam’ Transfer Functions H(ϖ/υ)

Interpolate to Obtain Principal Stresses Pmin & Pmax

Combine to Obtain Principal Stress Transfer Functions

MO

OR

ING

, Fx

MO

OR

ING

, Fz

MO

OR

ING

, My

VER

T A

CC

,

N

a z

. .

HO

RZ

AC

C N

a

x . .

BEN

DIN

G M

OM

ENT,

M B

mt

EXT,

PR

ESSU

RE,

P ex

t

INT,

PR

ESSU

RE,

P in

t

A1 A2 A3 A4 A5 A6 A7 A8

A H (ϖ/υ) A H (ϖ/υ) 2 2 A H (ϖ/υ) 3 3 A H (ϖ/υ) 4 4 A H (ϖ/υ) 5 5 A H (ϖ/υ) 6 6 A H (ϖ/υ) 7 7 A H (ϖ/υ) 8 8

H (ϖ/υ)

SEST

RA

STA

GE

3

PO

STF

EM

S

TAG

E 3

P

OS

TRE

SP

S

TAG

E 4

1 1

σ

Implementation P

OS

TRE

SP

S

TAG

E 4

Tz (secs) X

Stress

Spreading Function

Principal Stress Response Spectrum

Assume Rayleigh Distribution

Establish Probability of Occurrence

Σ for all points in scatter diagram

Total Distribution Average Cross Rate = T z

Select S-N Curve and Calculate Fatigue Life

No of Cycles

Total No of Cycles

σ

σ σ

σ

Rep

eat f

or a

ll P

oint

s in

Sca

tter D

iagr

am H (ϖ/υ)

2 σ

ϖ

HS

(m)

Fatigue Analysis

• Selection of S-N curve dependent on:

– Direction of principal stress relative to weld

– Mesh size

– Weld type

– CP protection

• SCF’s due to weld notch effect and local geometry

• Weibull or Rayleigh calculation

• Fracture mechanics

• Safety factors

Fatigue Analysis Example

R60 Cope hole

R60 Cope hole

Fatigue Analysis Example

Detailed Mesh at Location F

Fatigue Analysis Example

Location F Maximum Principal Stress S2

Fatigue Analysis Example

Location F Maximum Principal Stress S2

Fatigue Damage Calculations S-N curve: DEn-C-29

Fatigue life:= 1 / [8.499 E-3] = 115 years Fatigue Safety factor = 2 [dry, critical, inspectable & repairable] Target fatigue life = service life x safety factor = 25 x 2 = 50 years Hence, OK

No Description Damage per annum Ranking 1 Heave acceleration 4.324 E-16 3 2 Surge acceleration 7.182 E-18 5 3 Mooring force, Fx 3.385 E-16 4 4 Mooring Force, Fz 1.555 E-22 6 5 Mooring moment, My - - 6 Bending 7.741 E-3 1 7 External pressure 2.848 E-10 2 8 Internal pressure - -

Total 8.499 E-3

Performance

• Structural FE model 400,000 D.O.F.

• Analysis run on UNIX platform took 1.5 hours C.P.U. using new solver. Typically, SESTRA results file was 0.2 GBytes, POSTFEM database was 2 Gbytes.

• Analysis optimised by varying superelement hierarchy. Best performance achieved when minimum number of supernodes were carried forward to higher levels of hierarchy.

• Superelement approach ideally suited to FPSO fatigue problem. Solution times are faster (compared with analysis of one large model) and model can be built by team.

Use of F.E.A. at Irvine Engineering

Thank You

For further information on our designs and services please contact: Singapore Office: 39, Pandan Road, Singapore. 609281 Tel.: +65 91093201 +65 68539106 Malaysia Office: 11, Jalan 14/2, Taman Tar, Ampang, Salangor, Kuala Lumpur. 68000 Tel.: +60 342665601 Mob.: +60 123210824 +60 178810807 Irvine Engineering Pte Ltd - Registered Address: 60 Paya Lebar Road, #08-43 Paya Lebar Square, Singapore. 409051 Tel.: +65 68539106

Dubai Office: Jumeirah Lake Towers, P.O. Box: 643593, Cluster C, Goldcrest Executive Tower Office 706, Dubai. UAE. Tel.: + 971 4 447 0897 Fax: + 971 4 447 0896 Mob.: + 971 567451923 (Main Company Contact)

enquiries@irvineeng.com

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